如何防止Java扫描仪输入空格

Question

I'm trying to prevent the user from inputting spaces or no values. but nothing works. Even with no entered values program goes further without printing my error. What am I doing wrong? my code example

Scanner nameScan = new Scanner(System.in);
System.out.print("Input your name: ");

String newcomer = nameScan.nextLine();                 
player.setName(newcomer);
String userName = player.getName();
userName = userName.trim();


if (userName.length()==0) {
     System.out.println(" ");
     System.out.println("You have to set up a player name first... ");
     System.out.println(" ");                               
}
else {...

Answer 1

As @11thdimension said, you have to validate the input.

You can do something like:

if (newcomer.isEmpty()) {
    System.out.println("Please write something");
}

Or you can do a while loop and keep asking for a correct input.

Answer 2

Your code

if(username.length() == 0)

will not check whether the username contains space because space is counted towards the length of the String.

To check for empty String input(which may contain space(s)), you can do:

if("".equals(username.replaceAll("\\s+","")))   //or

if("".equals(username.trim())                   //or

if(username.isEmpty())                   

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Further more, you would want to use a do-while loop for validation instead of using an if-statement.